6.Al4 VI.A.4: SHIELDING Calculating the complex nature of events that take place when very high energy cosmic rays are incident on matter constitutes a major difficulty for the work in this section. To a first approximation, the only thing that counts for mass shielding is column mass density. For individual types of incident particles, however, this is not strictly true. In general light atoms are more effective in stopping charged particles by ionization and also project a higher geometric cross section per unit mass. However, radiative processes such as bremsstrahlung and pair production have a dE/dx proportional to z 2 . This suggests that high-Z materials, such as lead, might be desirable. Using atmospheric data to make numerical predictions as to effects behind denser mass shielding is a dubious practice because the IT+ mesons created by nuclear collisions do not get a chance to decay-in mass shields. Instead they collide with shielding nuclei, and produce heavy nuclear particles with high rbe's. Thus· the possible effects of secondary production in mass shielding are severe if one is trying to design for a low dosage environment. To ensure avoidance of such secondary cascade difficulties we need to know how to calculate dosages from arbitrary incident spectra with arbitrary shielding schemes. This problem {cosmic rays in mass shielding) has been worked by physicists since the turn of the century. It calls for a detailed simulation of actual events, requiring an extensive computer program. This program is beyond the scope of this study, but the problem can be defined, and an approximate method can propose an area density for the colony shielding. When the protons and nuclei described on Figure 6.A4, Table 6.~2, and Table 6.A3 are incident on matter, most of them have such high energies that the ionization losses {as described by the Bethe-Block equation) are saturated at about 2 MeV cm1 . This is a negligible energy loss rate for particles with energies in the BeV range and above. Every so often the incident particles hit a nucleus of the target material. When this happens, a number of particles are
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